Skin pigments undergo oxidation during absorption of light, in a reaction that has been linked with immediate tanning. As oxygen is consumed superoxide free radicals and hydrogen peroxide, which are important agents of oxygen toxicity, are generated. It has been hypothesized that this reaction is partly responsible for phototoxicity, the chronic effects of which include skin degeneration and cancer. New experiments are proposed in which electron spin resonance (ESR) experiments are used to monitor rapid light-promoted oxygen consumption in both isolated pigments and pigmented mammalian cells in vitro. A major objective of the proposed study is to understand the mode of generation of the superoxide radical and to obtain quantitative data (action spectra and quantum yields) for its formation. Many drugs selectively bind to pigment; the effects of such drugs (in particular photosensitizing agents and melanin precursors) on superoxide formation will be investigated. A second objective is to gain an understanding of the factors that influence toxicity towards pigmented cells in vitro. The effects of light upon the growth of pigmented and hypopigmented melanocytes (B16 melanoma cells) and non-melanocytes (CHO cells) will be compared with light-induced production of free radicals and oxygen consumption. Effects of free radical scavengers and specific enzymes will also be studied.